A known refrigerator uses the Brayton cycle as a refrigerating cycle and includes an expander-integrated compressor capable of cryogenic cooling. Further, in some cases, a non-contact type bearing such as a magnetic bearing is provided to support an output shaft of a motor for driving an expander-integrated compressor. A non-contact type bearing supports a rotational shaft such as an output shaft of a motor without making contact with the rotational shaft, and does not generate mechanical friction loss and abrasion with the rotational shaft, thus having a high durability compared with a roller bearing that supports a rotational shaft while making contact with the rotational shaft. Thus, for a motor that operates at a high rotation speed, for instance, a non-contact type bearing such as a magnetic bearing is provided to support an output shaft of the motor.
Patent Document 1 discloses an expander-integrated compressor provided with such a non-contact type bearing. The disclosed expander-integrated compressor is a magnetic-bearing type turbine compressor including a turbine impeller and a compressor impeller mounted to either end of a shaft, with a magnetic bearing pivotally supporting the shaft.
With a refrigerator provided with the expander-integrated compressor as disclosed in Patent Document 1, a part of expansion energy generated upon expansion of a fluid by the expander is recovered, and the recovered expansion energy is reused as rotational energy of a motor rotational shaft for driving the compressor. Thus, the power for driving the motor decreases, and the coefficient of performance (COP) of the refrigerator improves.
However, with the recent development in the cryogenic technologies, there is a need for further improvement of the coefficient of performance (COP) to further enhance the energy conservation property of refrigerators that use the Brayton cycle.
In an expander-integrated compressor, deterioration of the adiabatic efficiency of the expander and a decrease in COP of the refrigerator may be caused by heat penetration due to a refrigerant passing through a region formed inside an interior space of a casing due to a pressure difference between the compressor and the expander, leaking from the back-face side of the compressor toward the expander.
Meanwhile, a known refrigerator includes a plurality of compressors provided in more than one stages, to improve the compression ratio of the compressors to perform cryogenic cooling.
Patent Document 2 discloses a refrigerating air conditioner with two stages of compressors and expanders, which performs a two-stage compression and two-stage expansion cycle. In this device, an expander-integrated compressor, which integrally includes a high-stage compressor and the first and second expanders, is housed in a sealed vessel, which makes it no longer necessary to provide a seal between the compressors and the expanders.
In Patent Document 3, to solve the above problem of an expander-integrated compressor, leaking refrigerant is returned to the intake side or the discharge side of the compressor from an extraction line disposed in a casing.